Data recorded to a magnetic disk, optical disk, or other such recording medium is conventionally modulated to code words based on a coding method having specific coding rules to enable accurate reading during reproduction. One of these coding rules is to insert a synchronization code at the beginning of each frame, which is a data unit containing a known constant amount of data. The coded data containing the inserted synchronization code is then converted to a modulated signal, e.g., an NRZI (non-return to zero inverse) signal, which is recorded to the recording medium. The reproduction apparatus digital/analog converts the analog reproduction signal to obtain the NRZI signal, and then converts the NRZI signal to parallel data to obtain the code sequence from which the code words are extracted synchronized to the word clock. This word clock identifies the divisions between code words in the code sequence; the word clock phase is controlled based on the synchronization code. The extracted code words are then decoded and error correction processing is applied to obtain the processable data.
The synchronization code is also written as a code pattern that will not appear in the coded data recorded to the recording medium, and can therefore be discriminated from the other (non-synchronization code) data. More specifically, the coding rules normally define the maximum and minimum numbers of consecutive zeroes (0) bracketed by ones (1) that can appear in any one code word. The rules may further prohibit the maximum number of consecutive zeroes that can be in series separated by 1 at the conjunction of two or more words. As a result, the NRZI signal expressing the data code (non-synchronization code) part of the entire code according to said rules is a signal comprising maximum inversion interval T.sub.max and minimum inversion interval T.sub.min.
An example of this coding method is the EFM (eight-to-fourteen modulation) modulation method used with optical disks. In EFM-coded signals, code segments inverting at the maximum inversion interval T.sub.max will not appear consecutively in the NRZI signal expressing the non-synchronization code data. With EFM coding, the synchronization code therefore includes NRZI signals in which the maximum inversion interval T.sub.max occurs twice in succession.
FIG. 20 shows the pits and signal wave recorded to a conventional optical disk for data containing a synchronization code according to the prior art. As shown in FIG. 20, a synchronization code of signal length TS=2T.sub.max containing two consecutive maximum inversion intervals T.sub.max of the NRZI signal is added to the beginning of each frame of data other than the converted synchronization code. The data to which the synchronization code was added is converted to an NRZI signal, and the NRZI-modulated data is recorded to the optical disk track by forming a pit of a length determined by the HIGH level period of the NRZI signal, followed by a land (space) of a length determined by the LOW level period of the NRZI signal.
FIG. 21 is a timing chart showing the relationship between the reproduction signal for the data containing a synchronization code and reproduced from the recorded content of the optical disk, and the NRZI signal read from the reproduction signal. FIG. 21(a) shows the reproduction signal reproduced from the recorded content of the optical disk. FIG. 21(b) shows the read clock generated at pit interval T. FIG. 21(c) shows the NRZI signal digitized at a threshold value (or a threshold level) V0 according to the read clock. FIG. 21(d) shows the NRZI signal digitized at a variable threshold value (V0+.DELTA.V).
The reproduction signal reproduced from the recorded content of the optical disk is an analog signal as shown in FIG. 21(a). As shown in FIG. 21(b), this reproduction signal is then converted to a digital signal wherein values greater than or equal to the threshold value V0 are HIGH, and values below the threshold value V0 are LOW, based on the read clock of bit interval T. This threshold value may fluctuate, however, according to the low frequency component of the recorded content of the optical disk.
For example, when the signal level is .DELTA.V greater than the threshold value V0 as shown in FIG. 21(a), the HIGH level interval of the NRZI signal obtained from D/A conversion is shortened (=T.sub.max -2T), and the LOW level interval is lengthened (=T.sub.max +2T), as shown in FIG. 21(d). Though not shown in the figure, when the signal level is conversely .DELTA.V lower than the threshold value V0, the HIGH level interval of the NRZI signal obtained from D/A conversion is lengthened, and the LOW level interval is shortened. However, even when error occurs in the inversion interval of the NRZI signal obtained by D/A conversion as a result of threshold value fluctuations, the signal length TS of the synchronization code is still TS=2T.sub.max, and the other data can therefore be accurately discriminated.
There are cases, however, in which the maximum inversion interval T.sub.max of the NRZI signal is set to a large value by the rules of the coding method. When such coding methods are used, the signal length TS=2T.sub.max of the synchronization code naturally becomes longer, too. When long synchronization codes such as this are frequently recorded with the desired information as control data for reproduction, the ratio of desired information to control data drops in the information recorded to the recording medium. As a result, this interferes with improving the recording density of the data on an optical disk.
Recent research in optical coding and recording techniques strives to transmit as much information as possible while limiting the NRZI signal inversion period to less than the upper frequency limit of the transmission path. The result of this coding method is an increase in the maximum inversion interval T.sub.max. For example, the maximum inversion interval T.sub.max of the modulated NRZI signal is defined as 14T by the coding rules of the 8-15 conversion method whereby eight data bits other than the synchronization code are converted to code words expressed by a 15-bit pattern. If the signal length TS of the synchronization code remains 2T.sub.max and the coding benefits described above are retained, the synchronization code is expressed by an NRZI signal of signal length TS=28T, significantly longer than the signal length of the synchronization code generally used today.